Powder injection molding is an emerging technology for making metal parts. One of the main advantages of using the powder injection molding method is that it provides a quick and relatively simple way to fabricate small, high-precision three-dimensional parts with relatively complicated external features. Typically, the powder injection molding process involves the steps of first mixing a metal powder with a multi-component binder composition, then forming a green compact from the metal powder/binder mixture via an injection molding process. The green compact is then subject to debinding (by firing) and sintering steps until the sintered body has taken its permanent predetermined form. The use of multiple components in the binder composition allows the various components to be sacrificed at different stages to avoid deformation or collapse of the green compact during the debinding step, thus ensures the dimensional integrity of the final product.
The most commonly used binder composition used in the metal powder injection molding typically contains two or more polymeric components and an appropriate amount of oil or wax. Fatty acids are conventionally added to the binder composition as a surface active agent and/or plasticizer. The binder composition then blendes with metallic powder to form an injection/molding composition. The polymeric components contained in the binder composition typically include non-crystalline polymers such as polystyrene, and crystalline polymers such as polypropylene. Because of the different properties among the various polymeric components in the binder composition, compatibility often becomes a problem. And the incompatibility between or among the polymeric components can result in inhomogeneity in the binder composition and adversely affect the dimensional integrity as well as the precise shape of the sintered parts, making the precision control the final dimension and shape of the final products difficult.
U.S. Pat. No. 4,158,688 discloses a sacrificial binder composition for molding particulate solids, including powder of lithium-modified beta-alumina, into sintered products. The binder composition disclosed in the '688 patent comprises a block copolymer and a plasticizer. The block copolymer is represented by the following formula: X-[B(AB).sub..eta. A].sub..eta.', wherein "A" is a linear or branched polymer that is glassy or crystalline at room temperature, "B" is a polymer that behaves as an elastomer at processing temperature, ".eta." is 0 or a positive integer, ".eta.'" is a positive integer greater than 2, and "X" is either "A" or "B". The plasticizer may be oil, wax, or oil and wax. The copolymer disclosed in the '688 patent does not address any compatibility problem when multiple polymeric binder components are used.
U.S. Pat. No. 4,283,360 discloses a process for producing molded ceramic metal by which a solvent-soluble resin and a solvent-insoluble resin, a ceramic or metallic powder and a plasticizer are blended and molded. The molded product is treated with an organic solvent to dissolve the solvent-soluble resin. Then, the treated product is fired to obtain a molded ceramic or metal product. Again, the potential incompatibility problem was not addressed between the solvent-soluble and solvent-insoluble resins, and the dimensional integrity of the sintered product can be adversely affected as a result of such incompatibility.